JPH11196580A - Switching power supply device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To set the crest value of an AC voltage obtained from a predetermined winding by the operation of a switching device at a required value and avoid a trouble in the operation of a load such as a fluorescent display tube, etc. SOLUTION: One end of the secondary winding N2 of a transformer T is connected to a predetermined AC output terminal 3a through an inductance L1. A series circuit consisting of a diode D2 and a constant voltage diode DR is connected between the output terminal 3a to which the inductance L1 is connected and the other AC output terminal 3b. An impedance circuit 5a is composed of the inductance L1 and a voltage clamp circuit 6a is composed of the series circuit of the diode D2 and the constant voltage diode DR. The crest value of a voltage in a flyback direction which is generated in the secondary winding N2 is limited to be a proper value by the Zener voltage of the constant voltage diode DR of the voltage clamp circuit 6a and a trouble in the operation of a load such as a fluorescent display tube, etc., is avoided.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a switching power supply device for obtaining a plurality of output voltages, and more particularly to operation of a load, specifically, display control of a fluorescent display tube, using an AC output voltage obtained from a predetermined winding of a transformer. The present invention relates to a technique for preventing a problem from occurring.

[0002]

2. Description of the Related Art In recent years, electronic equipment often requires a multi-output type power supply as a power supply incorporated in the electronic equipment. This is because various electronic devices and active components developed in recent years often require a plurality of drive voltages or control voltages having different voltage values. Here, a fluorescent display tube is assumed as an electronic device whose operation requires a plurality of drive voltages and control voltages. As a power supply device for supplying electric power to the fluorescent display tube, there has conventionally been a multi-output type switching power supply device having a circuit configuration as shown in FIG. The general operation of the switching power supply device shown in FIG. 7 was as follows. In FIG. 7, reference numerals 1 and 2 indicate an input terminal and an output terminal (DC) on the high potential side, respectively. The input terminal and output terminal on the reference potential side shall be connected to ground,
The illustration is omitted.

When the input voltage V _IN is applied to the input terminal 1, the circuit shown in FIG. 7 starts operating. At this time, the switching transistor Q1 turns on or off alternately in response to the pulse signal from the control circuit 4, and interrupts the current passing through the primary winding N1. A voltage is generated in each winding of the transformer T by repeating the intermittent current. Here, a combined voltage of the voltage generated in the primary winding N1 and the input voltage V _IN appears at a connection point between the primary winding N1 and the anode of the diode D1. This combined voltage is a diode D
1. Rectified and smoothed by the capacitor C2, and guided to the first output terminal 2 as a DC output voltage V _O2 . on the other hand,
The voltage generated in the secondary winding N2 is guided between the second output terminals 3a and 3b as an AC output voltage _VO3 . Incidentally, when the load is a fluorescent display tube, this DC output voltage V _O2 is supplied to a drive circuit for controlling display of the fluorescent display tube, and the AC output voltage V _O3 is supplied to a filament of the fluorescent display tube. You.

Here, the output voltage V _O2 is determined by the resistance R
2, the divided voltage of the voltage V _O2 appearing at the connection point of R3 is detected, and the switching transistor Q
1 is controlled to be constant by the control circuit 4 that changes the on-duty of 1. On the other hand, if the AC output voltage V _O3 is constant, the input voltage V _IN , the state of the load connected to each output terminal (2, 3a to 3c) and the DC output voltage V _O2 are constant. It is kept almost constant by the magnetic coupling between the winding N1 and the secondary winding N2.

[0005]

In the circuit shown in FIG.
AC voltage V _O3 appearing at the output terminal 3a~3c secondary winding N
2 is obtained from the voltage generated. Since the primary winding N1 and the secondary winding N2 are magnetically coupled, this secondary winding N2
Has a waveform similar to that of the voltage generated across the primary winding N1. Here, the voltage waveform generated between the terminals of the primary winding N1 is shown in FIG.
It becomes as shown in. That is, the period T1 of the switching transistor Q1 is turned on, its peak value appears a voltage V _P2 equal to approximately the input voltage V _IN, the switching transistor Q1 is in a period T2 in the OFF state, the polarity of the voltage V _P2 in contrast, the peak value is approximately (DC voltage V _O2) - the voltage V _P1 as the magnitude of (the input voltage V _iN) appears.

The switching power supply having the circuit configuration shown in FIG. 7 employs a PWM control method for stabilizing control of the output voltage V _O2 , so that the periods T 1 and T 2 in which the switching transistor Q 1 is on or off are load-controlled. It changes depending on the state of. However, in normal operation, as shown in FIG. 8, towards the period T1 in the ON state becomes shorter than the period T2 in the OFF state, also across the position equivalent to the AC voltage zero volts, the voltage V _P2 Is generally low, and the peak value of the voltage _VP1 is generally high.
Therefore, the AC voltage V appearing at the output terminals 3a to 3c
_O3 has a distorted voltage waveform that is vertically asymmetric. When a filament of a fluorescent display tube is connected as a load to the output terminals 3a and 3b of the circuit of FIG. 7 and an asymmetric vertical AC voltage V _O3 is supplied to the filament, the following problems may occur. .

That is, although it depends on the specifications of the power supply device and the fluorescent display tube, for example, the DC output voltage V _O2 is _replaced with the input voltage V _IN.
Consider a case where a much higher voltage value is required.
In such a case, the period T2 in the voltage waveform of FIG. 8 becomes shorter, and at the same time, the peak value of the voltage _VP1 becomes higher. As a result, the difference between the periods T1 and T2 increases, and the negative voltage V
The peak value of _P2 and the positive voltage V _P1 becomes different to the extreme. Then, the fluorescent display tube receiving the supply of the output voltage V _O3 having a voltage waveform similar to the voltages V _P2 and V _P1 does not completely erase the display even if it receives the display erase signal on its grid, or vice versa. In some cases, the display may not be performed even if a lighting display signal is received. Therefore, the present invention provides a switching power supply device that enables a peak value of an AC voltage obtained from a predetermined winding to be set to an appropriate value in accordance with the operation of a switching element, and does not cause a problem in operation of a load such as a fluorescent display tube. The purpose is to provide.

[0008]

According to the present invention, there is provided a switching device comprising a switching element, a control circuit, and an inductance component provided with a plurality of windings, the switching element being driven by the control circuit. In a switching power supply device in which a current flowing through a first winding of an inductance component is intermittently turned on and off to obtain a plurality of output voltages from voltages generated in each winding of the inductance component, a second winding of the inductance component is obtained. The impedance circuit connected between the predetermined winding position and the output terminal, and connected between predetermined output terminals including the output terminal connected to the impedance circuit, the peak value of the voltage appearing between the output terminals And a limiting voltage clamp circuit.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION Switching element and transformer
The secondary winding is connected, and the switching element interrupts the current flowing through the primary winding. One end of the secondary winding of the transformer is connected to a predetermined AC output terminal via an impedance circuit. The impedance circuit may be connected to a tap provided at a predetermined winding position instead of one end of the secondary winding. Then, a voltage clamp circuit is connected between predetermined output terminals including an output terminal to which the impedance circuit is connected so as to limit a peak value of a voltage appearing when the switching element is turned off. Here, specifically, the impedance circuit is configured by an inductance element or a capacitance element. Further, the voltage clamp circuit is a series circuit of a diode and a constant voltage diode,
Alternatively, it is constituted by a bidirectional constant voltage diode.
This voltage clamp circuit limits the peak value of one of the AC voltages to a desired value, thereby preventing a problem in operation of a load such as a fluorescent display tube.

[0010]

FIG. 1 shows a circuit of a switching power supply according to a first embodiment of the present invention, which does not cause any trouble in the operation of a load such as a fluorescent display tube. The configuration of the circuit shown in FIG. 1 is as follows. First, the primary winding N1 of the transformer T and the switching transistor Q1 are connected in series between the input terminal 1 and the ground, and the primary winding N1
Connection point of the switching transistor Q1 and the output terminal 2
And a smoothing capacitor C2 connected between the output terminal 2 and the ground to form a boost converter. C1 between the input terminal 1 and the ground is a filter capacitor. A series circuit of a resistor R2 and a resistor R3 is connected between the output terminal 2 and the ground,
The connection point between the resistors R2 and R3 is connected to the voltage detection terminal FB of the control circuit 4. The pulse output terminal PO of the control circuit 4 is connected to a power supply terminal V _{CC at} the base of the switching transistor Q1.
Is connected to the input terminal 1 via the resistor R1 and to the ground terminal GND.
Are connected to earth. The circuit configuration described above is the same as the circuit in FIG.

Then, one end of the secondary winding N2 of the transformer T is connected to the output terminal 3a via the inductance L1,
The other end and the tap of the secondary winding N2 are connected to the output terminal 3b, respectively.
And 3c directly. A series circuit of a constant voltage diode DR and a diode D2 is connected between the output terminal 3a to which the inductance L1 is connected and another output terminal 3b. The inductance L1 forms an impedance circuit 5a, and the series circuit of the constant voltage diode DR and the diode D2 forms a voltage clamp circuit 6a. The circuit of FIG. 1 having such a configuration is almost the same in basic operation as the circuit of FIG. 7, but the output voltage V _O3 is determined by the impedance circuit 5a and the voltage clamp circuit 6a as follows. Be affected.

[0012] The switching transistor Q1 when turned off, a high flyback voltage wave height value in the secondary winding N2 which corresponds to the voltage V _P1 in FIG. 8 is generated. But,
This flyback voltage is applied in the forward direction of the diode D2 forming the voltage clamp circuit 6a. Therefore, peak value of the flyback voltage at the output terminal 3a, 3b of the position is limited to a value of the combined forward voltage drop V _F of the Zener voltage V _z and the diode D2 of the substantially constant-voltage diode DR. Here, the impedance circuit 5a generates a potential difference between both ends, and the action of the peak value limitation by the voltage clamp circuit 6a is applied to the DC output voltage V _O2 of the output terminal 2 through the magnetic coupling between the secondary winding N2 and the primary winding N1. Prevent adverse effects. In this way, by limiting the peak value of the flyback voltage corresponding to the voltage V _P1 of FIG. 8 to a constant value, even if the load is a fluorescent display tube, even if a display erase signal is received on the grid, it is completely completed. It is possible to prevent such a phenomenon that the display is not erased or, conversely, the display is not performed even when the signal of the lighting display is received.

FIGS. 2, 3, and 4 show second, third, and fourth embodiments of the switching power supply according to the present invention, respectively. In the drawings of the embodiment after FIG. 2, only the circuit portion corresponding to the circuit on the output side from the secondary winding N2 of the transformer T in the circuit of FIG. 1 is shown. The circuit of FIG.
In addition to the impedance circuit 5a, the impedance circuit 5b is connected between the other end of the secondary winding N2 and the output terminal 2b. On the other hand, FIG. 3 shows a circuit configuration in which the voltage clamp circuit 6a is connected between the output terminals 3a and 3c while the impedance circuit 5a is the same as in FIG. FIG. 4 shows a circuit configuration in which an impedance circuit 5c having an inductance L3 is connected between the tap of the secondary winding N2 and the output terminal 3c, and a voltage clamp circuit 6a is connected between the output terminals 3b and 3c. .

Each of the circuits shown in FIGS. 2, 3 and 4 has an impedance circuit (5a to 5a) when a plurality of output lines are drawn from the secondary winding N2 by taps or the like.
5C shows a modification of the connection configuration between the voltage clamp circuit (5c) and the voltage clamp circuit (6a). Each circuit has at least one impedance circuit, and a voltage clamp circuit is connected between an output terminal to which the impedance circuit is connected and another terminal. The functions of the impedance circuit and the voltage clamp circuit in each circuit are substantially the same as those of the circuit of FIG.

FIGS. 5 and 6 show fifth and sixth embodiments of the switching power supply unit according to the present invention, respectively.
In the embodiment of FIGS. 1 to 4, the voltage clamp circuit 6a
Is composed of a series circuit of a constant voltage diode DR and a diode D2. On the other hand, the circuit of FIG. 5 shows a case where a bidirectional constant voltage diode DDR is connected between the output terminals 3a and 3c, and the voltage clamp circuit 6b is configured by the bidirectional constant voltage diode DDR. In the embodiments shown in FIGS. 1 to 5, all the impedance circuits are constituted by the inductances L1 to L3. On the other hand, the circuit in FIG. 6 shows a case where a capacitor C3 is connected between one end of the secondary winding N2 and the output terminal 3a, and the impedance circuit 5d is configured by the capacitor C3.

Each of the circuits shown in FIGS. 5 and 6 is an example of a circuit in which an impedance circuit and a voltage clamp circuit are constituted by elements different from those in FIG. Although the elements used are different, substantially the same operations and effects as those of the impedance circuit and the voltage clamp circuit in the circuit of FIG. 1 can be expected by the circuits of FIGS. Naturally, the connection configuration between the impedance circuit and the voltage clamp circuit shown in FIGS.
Modifications as shown in FIGS. 3 and 4 are also possible.

Although the first to sixth embodiments of the present invention have been described above, the circuit configuration of the switching power supply according to the present invention is not limited to the circuit configurations shown in FIGS. For example, another secondary winding may be provided in the transformer T, and a voltage generated in the secondary winding may be rectified and smoothed to obtain a DC output voltage V _O2 . Further, in the circuit of FIG. 1, the primary winding N1, the switching transistor Q1, the diode D1, and the smoothing capacitor C2 have a circuit configuration of a boost converter. However, the circuit configuration may be modified to be a circuit configuration of a polarity inversion converter. Absent. Further, in the description of the present invention, a problem with the operation of the fluorescent display tube of the conventional power supply device was pointed out, but the target load to which the power supply device to which the present invention is applied supplies power is limited to the fluorescent display tube. Not something.

[0018]

As described above, according to the present invention, at least one impedance circuit is provided between a winding from which an AC voltage is obtained and an output terminal, and the output terminal connected to the impedance circuit is connected to another terminal. And a voltage clamp circuit connected between them. According to this configuration, the peak value of the voltage in the flyback direction of the AC voltage can be limited or set to a desired value. As a result, when alternating voltages that are not symmetrical in the vertical direction are supplied to a load such as a fluorescent display tube, it is possible to prevent occurrence of a phenomenon that is inconvenient for driving the load. Therefore, it is possible to provide a switching power supply device that does not cause a problem in operation of a load such as a fluorescent display tube.

[Brief description of the drawings]

FIG. 1 is a circuit diagram of a first embodiment of a switching power supply according to the present invention.

FIG. 2 is a partial circuit diagram of a second embodiment of the switching power supply device according to the present invention.

FIG. 3 is a partial circuit diagram of a third embodiment of the switching power supply according to the present invention.

FIG. 4 is a partial circuit diagram of a fourth embodiment of the switching power supply according to the present invention.

FIG. 5 is a partial circuit diagram of a fifth embodiment of the switching power supply according to the present invention.

FIG. 6 is a partial circuit diagram of a sixth embodiment of the switching power supply according to the present invention.

FIG. 7 is a circuit diagram of an example of a conventional switching power supply for obtaining a plurality of voltages including an AC voltage.

FIG. 8 is a waveform diagram of a voltage appearing on a primary winding of the circuit shown in FIG. 7;

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 Input terminal 2 Output terminal (DC) 3a-3c Output terminal (AC) 4 Control circuit 5a-5d Impedance circuit 6a, 6b Voltage clamp circuit T transformer Q1 Switching transistor N1 Primary winding as first winding N2 First winding Secondary winding as winding 2 V _IN input voltage V _O2 output voltage (DC) V _O3 output voltage (AC)

Claims (5)

[Claims] A switching element, a control circuit, and an inductance component provided with a plurality of windings, wherein the switching element driven by the control circuit turns on and off the first winding of the inductance component. In a switching power supply device that intermits a flowing current and obtains a plurality of output voltages from a voltage generated in each winding of the inductance component, a predetermined winding position of a second winding of the inductance component, an output terminal, And a voltage clamp circuit connected between predetermined output terminals including the output terminal to which the impedance circuit is connected, and limiting a peak value of a voltage appearing between the output terminals. A switching power supply device. 2. The switching power supply device according to claim 1, wherein said impedance circuit comprises an inductance element. 3. The switching power supply device according to claim 1, wherein said impedance circuit comprises a capacitive element. 4. The switching power supply according to claim 1, wherein the voltage clamp circuit comprises a series circuit of a diode and a constant voltage diode. 5. The switching power supply device according to claim 1, wherein said voltage clamp circuit comprises a bidirectional constant voltage diode.